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Walter Landry wrote:<br>
<blockquote
cite="mid:20090507.155710.131509470607391773.walter@geodynamics.org"
type="cite">
<pre wrap="">
The "tolerance" variable in the Stokes_SLE_UzawaSolver controls both
the accuracy of the overall Uzawa iteration as well as the individual
(linear or direct) solves within that solution. </pre>
</blockquote>
If by "tolerance" you mean the "linearTolerance" variable then yes I
understand this.<br>
<blockquote
cite="mid:20090507.155710.131509470607391773.walter@geodynamics.org"
type="cite">
<pre wrap="">You can not specify
the accuracy of the direct solver because there is no iteration. It
directly inverts the matrix.
</pre>
</blockquote>
Yes, I understand this too.<br>
<blockquote
cite="mid:20090507.155710.131509470607391773.walter@geodynamics.org"
type="cite">
<pre wrap="">
Most of the time, setting the tolerance for linear solves to be very
small should give you the same result as a direct solve. </pre>
</blockquote>
I disagree as I have never seen evidence for this behavior. No matter
what linearTolerence is set to I ALWAYS require more nonlinear
iterations with the iterative solver than the direct solver. What I
was asking was whether it would be a good idea to control the tolerance
of that specific part within the Uzawa iteration the iterative solver
solves, separately than "linearTolerance" (which, as I understand
controls the tolerance of the whole Uzawa iteration). Basically, I'm
wondering if there is a (simple) way the code could be modified to
drive specifically the iterative solver towards more accurate
solutions. (i'm beginning to realize this may be a rhetorical question)<br>
<br>
What step within the Uzawa scheme is the direct/iterative solver
solving?<br>
<blockquote
cite="mid:20090507.155710.131509470607391773.walter@geodynamics.org"
type="cite">
<pre wrap="">However, if
the matrix is singular or nearly so, roundoff error will give you the
wrong answer.
Multigrid will probably have the same problems. Better
preconditioning might help.
</pre>
</blockquote>
<blockquote
cite="mid:20090507.155710.131509470607391773.walter@geodynamics.org"
type="cite">
<pre wrap="">
Cheers,
Walter Landry
<a class="moz-txt-link-abbreviated" href="mailto:walter@geodynamics.org">walter@geodynamics.org</a>
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</pre>
</blockquote>
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<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;">=====================================<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;">Prof.
Garrett Ito<span style=""> </span><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;">SOEST,
Geology & Geophysics<span style=""> </span><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;">1680
East-West Rd., POST 810<span style=""> </span><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;">Univ.
of
Hawaii<span style=""> </span><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;">Honolulu,
HI
96822 USA<span style=""> </span><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;">808-956-9717<span
style=""> </span><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;"><a class="moz-txt-link-abbreviated" href="mailto:gito@hawaii.edu">gito@hawaii.edu</a>
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;"><a class="moz-txt-link-abbreviated" href="http://www.soest.hawaii.edu/GG/FACULTY/ITO">www.soest.hawaii.edu/GG/FACULTY/ITO</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size: 9pt; font-family: Arial;">======================================<o:p></o:p></span></p>
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